1. Field of the Invention
This invention relates to a power transistor overcurrent protection circuit, and more particularly to an overcurrent protection circuit of a power transistor of the voltage-driven type such as an IGBT or a MOSFET, that provides high speed current limiting of overcurrent produced by load short-circuiting etc., thereby providing breaking and protection in a safe operating range.
2. Description of the Related Art
IGBTs (insulated gate bipolar transistors) are widely known as voltage-driven power transistors. There has been a rapid expansion of the field of application of IGBTs on account of their low ON voltage, little drive power with MOS gate construction, and comparatively fast switching. ON voltage and switching speed characteristics are mutually antagonistic, and there is an unceasing research effort aimed at improving the trade-off between these so as to provide better device performance.
Characteristic C in FIG. 9a is the ON voltage characteristic of a third generation or later IGBT predicted from such studies. The ON voltage characteristic A of a first generation IGBT is shown for comparison. The characteristic B of the currently-used second generation IGBT is also shown. The collector current IC is expressed as a percentage, taking the rated current of the respective IGBT as 100.
As can be seen from these characteristics, when the collector-emitter voltage VCE rises under load short-circuit conditions, a collector current IC that is extremely larger than the rated current flows. In the case of a first-generation IGBT, this overcurrent is some 6 to 8 times the rated current, while in the case of a second-generation IGBT, it is some 10 to 12 times.
Third and subsequent generation IGBTs that are presently under study and have DRAM-class patterning of the am order, and other improvements. These result in an overcurrent characteristic C of some 15 to 20 times the rated current.
With such a large overcurrent, high speed current limiting and breaking are difficult, making overcurrent protection of the device difficult. Furthermore, with such a large overcurrent, even if high speed current limiting and circuit breaking are made, they result in excessive surge voltage, which makes safe protection difficult, and makes difficult overcurrent protection of the device.
Protection of conventional IGBTs on load short-circuiting is described below. FIG. 9b shows a typical main circuit layout using IGBTs. In this device, an electric motor 3 is driven by AC voltage converted from the DC voltage of a DC voltage source 1, using a bridge-type converter (inverter) 2 consisting of IGBTs 21 to 26. In such a device, when short-circuiting occurs across the terminals of the load (electric motor), a short-circuit current flows through the positive-side and the negative-side IGBTs. Short-circuit current can likewise arise if an ON signal (due to noise or incorrect operation) is input simultaneously to the positive-side and negative-side IGBTs in the same arm.
The amount of time that an IGBT can withstand such a short-circuit condition is 10 to 20 .mu.s at a voltage of 80% of the device rated voltage in the case of present IGBTs. This implies that short-circuit protection must be provided such that overcurrent is detected and current limitation and circuit breaking are applied within 7.5 to 10 .mu.s.
FIG. 10b is a characteristic graph of the short-circuit withstand capability of an IGBT showing the relationship between a collector current IC flowing on short-circuit under fixed collector-emitter voltage VCE and a withstand time tw. The test circuit thereof is also shown in FIG. 10a. As can be seen from this characteristic, collector current IC and withstand time tw have a practically constant-power relationship. Accordingly, the withstand time tw is shortened if collector current IC is increased by load short-circuiting etc, so a high speed protective action is required.
Various methods have accordingly been proposed of lengthening the apparent short-circuit withstand time by restricting the short-circuit current, utilizing the transistor action of the IGBT by lowering the gate voltage of the IGBT when overcurrent due to load short-circuiting etc is detected (Japanese patent disclosure (Kokai) No. P61-251323, Nov. 8, 1986).
The circuit shown in FIG. 11a was disclosed at a learned society (470, National Congress of the Institute of Electrical Engineers of Japan, 1992). A current sensing IGBT 4b is provided that detects the current of main IGBT 4a. When it exceeds a specific current the IGBT gate voltage is lowered by NLU (non latch-up) circuit 50, suppressing the short-circuit current.
One example of the NLU circuit 50 is shown in FIG. 11d, where 52 and 53 are resistors, and 54 is a MOSFET. In FIG. 11d, when the voltage drop of resistor 52 produced by the emitter current of current sensing IGBT 4b exceeds the gate threshold voltage of MOSFET 54, current flows to the drain of MOSFET 54 through a gate resistor 51 thereby lowering the gate voltage of the IGBT 4b. Alternatively, NLU circuit 50 could be a circuit as shown in FIG. 11e. In this circuit, MOSFET 54 is replaced by a bipolar transistor 55.
With such a circuit construction, the short-circuit current can be restricted as shown in FIG. 11c, and an apparent lengthening of the short-circuit withstand time can be achieved as shown in FIG. 11b by increasing the ON resistance of IGBT by lowering the gate voltage of the IGBT on overcurrent.
Although the short-circuit withstand capability of the IGBT is increased by the prior art method of FIG. 11, it is subject to the following problems. A separate circuit is required to turn off the signal for driving the IGBT bridge when a short-circuit fault is detected. Specifically, since it is designed that the short-circuit current is restricted to about 200%, detection of short-circuit fault is made difficult by the above-described overcurrent detection.
It is necessary to add a circuit to turn the drive signal off, by means of a separate short-circuit fault detection circuit or the like, which detects for example the situation that the voltage VCE of the IGBT remains high irrespective of application of an ON signal. This makes the circuitry further complicated.
Also, since the gate voltage is different even for the same drain current of FET 54 of FIG. 11d, depending on the value Rg of the gate resistance 51, the current limiting value ICL changes as shown in FIG. 12 depending on the magnitude of the value Rg.
Reduction in the variation of the current limiting value ICL can be achieved by increasing the gain of the drain current with respect to the current detection value of FIG. 11d. However, there is a limit to the extent to which this can be achieved, in that, if this gain is raised too far, MOSFET 54 performs a switching action rather than an analogue action, so that the gate of IGBT 4 drops to around zero and the current of the IGBT oscillates between the ON and OFF conditions. This results in the characteristic shown in FIG. 12.
A drawback is therefore that, if the value Rg is made small in order to speed up the switching of the IGBT, the short-circuit current limiting value ICL rises, diminishing the short-circuit withstand time.
A further problem is the generation of oscillation when IGBTs are connected in parallel. Specifically, if there is some difference in the operating levels of the current limiting functions of respective IGBTs, when the current limiting function of one IGBT is triggered, the current which is thus restricted will shift to another IGBT triggering the current limiting function of that IGBT. Repetition of this process results in a kind of oscillatory condition which risks damaging the IGBTs.
Also, although the operation of the IGBT current limiting function of the circuit shown in FIG. 11a is very fast, when an IGBT is used in antiparallel connection with a diode there are the following problems.
Specifically, as shown in the circuit of FIG. 13, discharge current iD flows to diode 21D from load inductor 3L, and when IGBT 24 turns on, as shown by the waveform diagram of FIG. 13, a peak current of about 1.5 to 2 times the load current iD flows in the rising portion of current IC of IGBT 24. The shaded portion of this current waveform is the recovery current of diode 21D and there is a characteristic that it increases as the withstand voltage of the diode is increased, furthermore it increases with rise of temperature. If the current limiting value is lower than this peak current, the turn-on loss of the IGBT is enormously increased. Therefore, assuming that discharge current iD is used up to the rated current of the IGBT, the peak current becomes about 200% of the rated current. Taking into account the need for a margin, the current limiting value must therefore be set at at least 250% of the rated current. This results in the problem of the short-circuit withstand time being shortened.
Also, if the value Rg of gate resistor 51 is reduced in order to speed up the switching action, a current limiting value of about 500 to 600% of the rated current may be required. This leads to the drawback of protection harmonization being difficult to achieve.
A further problem is that, even in the system, as shown in FIG. 11, where the NLU circuit is provided, although the peak value of the current is limited, the high speed of breaking of the current results in the safe operation of the IGBT being exceeded due to the surge voltage (-L0.dIC/dt) produced by the stray inductance L0, with the risk of deterioration of the device.
FIG. 10c shows an example of the safe operating region of an IGBT. It is necessary to lower the surge voltage VCEP as the current IC is increased. In particular, if the capacity of the converter is increased, the current on fault also becomes large, but, since constructional considerations prevent the stray inductance from being decreased, the surge voltage will tend to show a corresponding increase.
Accordingly, in order to lower the surge voltage, the only expedient available is to lower the rate of current change dIC/dt.
Finally, the current, which has been limited to 250% to 500% must be subjected to a breaking operation by turning off the drive signal. The surge voltage that is thereby generated is determined by (-L0.dIC/dt). If this surge voltage passes outside the reverse bias safe operating region of the transistor, the transistor will be permanently damaged. In the case of breaking a current of 200%, the safe operating region of the IGBT falls to about 80% of its rated voltage.
In particular, if the capacity of the converter is increased, stray inductance L0 also tends to increase, and, since current IC is proportional to the capacity, there is a considerable increase in surge voltage. This results in the snubber circuit (surge absorption circuit) occupying more area and representing more cost than the main devices of the transistors.